1. Field of the Invention
The present invention relates to a technology for learning, as a first-arrival port, a port at which an identical packet that is transmitted through a plurality of paths by other relay apparatus arrives first in a relay apparatus that controls a path of a packet in a network through which packets are distributed.
2. Description of the Related Art
With a spread of a broadband Ethernet (Registered trademark) service, an advance of the Ethernet (Registered trademark) technology for a broadband network, such as a metro area network (MAN) and a wide area network (WAN), has become outstanding. As a relay apparatus that relays a data packet distributed on such a broadband network, a layer 2 switch (L2 switch) has got a lot of attention recently.
The L2 switch has a big merit of being less expensive compared to a router and can be used for all upper-layer protocols, which may make a further spread in the future. However, because the L2 switch is a relay apparatus originally intended for a local area network (LAN) without considering an to the broadband network such as the MAN, it has a few problems in terms of a system.
For example, when a path is controlled so that a packet is transmitted through an optimum path in a network in which a plurality of L2 switches are mutually connected, it may not be possible to select the optimum path depending on a physical configuration of the network even with employing a spanning-tree protocol (STP) defined as a network redundancy system of the Ethernet (Registered trademark) or a multiple spanning-tree protocol (MSTP) that is an extended system of the STP, which causes a problem of concentration of a network load on a specific path.
In addition, in a network employing the STP or the MSTP, the maximum hop count is limited to about 20 due to a limitation of the STP, which causes another problem of scalability implying that it is not possible to build a bigger size network.
To cope with the above problems with the STP and the MSTP, a so-called first-arrival-port learning method (hereinafter, “a first-arrival learning method”) has been proposed (see, for example, Japanese Patent Application Laid-Open No. H3-162035 and Japanese Patent Application Laid-Open No. H6-37764). In the first-arrival learning method, an optimum path for transferring a packet is selected by learning a port having a shortest delay of the packet.
In other words, when an identical packet transmitted through a plurality of paths on a network by other relay apparatus (hereinafter, “an identical packet”) arrives at a plurality of ports of a relay apparatus provided at a certain node, a port that first received the identical packet is learned as a first-arrival port for-the identical packet. Then, a flooding (simultaneous distribution) of the identical packet arrived at the first-arrival port is performed to other ports, and at the same time, the identical packets arrived at the other ports are discarded.
However, the method disclosed in Japanese Patent Application Laid-Open No. H3-162035 and Japanese Patent Application Laid-Open No. H6-37764 is a basic concept of the first-arrival learning method, and only the basic concept cannot build a practical relay apparatus.
Because the first-arrival learning method is a method in which “a port at which a packet arrives first is considered as the first-arrival port”, packets are transferred using a port having the shortest delay of the packet. However, the delay includes a so-called “queuing delay” that is apt to be influenced by a temporary congestion. Therefore, if the first-arrival port is learned when the queuing delay is temporarily increased on a path that is supposed to be the optimum path, a long path would be selected as the optimum path, which means that it is a problem in the first-arrival learning method how to ensure an accuracy of the optimum path.
In addition, when there is a plurality of paths having substantially same delays, the optimum path tends to be changed very often in the first-arrival learning method, because the path having the shortest delay is changed at frequent intervals due to an influence of the queuing delay that occurs by the temporary congestion. The frequent change of the optimum path (the first-arrival path) is not desirable because it may cause a reversal of a packet order, which means that it is another problem in the first-arrival learning method how to ensure a stability of the optimum path.
There is still another problem in the first-arrival learning method that, even when a failure occurs in a path (a link) or a relay apparatus (a node) that is related to the optimum path, the transfer of the packet is continuously performed to the optimum path once learned. Although it is possible to learn information again when a valid duration is expired with a setting of the valid duration for learned information, the transfer of the packet is still continued to a failed path until the valid duration passes off, which means that it is still another problem in the first-arrival learning method how to perform a reselection of the optimum path when a failure occurs.
From the above points of view, it is a major challenge how to realize a first-arrival leaning method having a capability of ensuring accuracy and stability of the optimum path, and a relay apparatus and a program for the relay apparatus to which the first-arrival leaning method is applied.